Nacreous pigments, also known as pearlescent or effect pigments, exhibit pearl-like and/or iridescent effects upon the transmission and reflection of light therethrough. As is well known in the art, the characteristics of such pigments depends upon optical interference phenomena as more fully described, for example, in xe2x80x9cThe Properties of Nacreous Pigmentsxe2x80x9d, Greenstein and Miller, Technical Papers, Vol. XIII, Annual Technical Conference, Society of Plastic Engineers, May 1967.
Nacreous pigments are conventionally formulated for use in suspensions of light transmitting resinous media which can be applied by dipping or spraying operations to form plastic coatings or by extruding, molding, casting or like techniques to provide solid plastic articles incorporating such pigments. Nacreous pigments so utilized should have indexes of refraction which differ from the suspending media because the pearly or nacreous effect displayed by such pigments depends upon the difference between the index of refraction of the oriented, plate-like pigment particles and the index of refraction of the medium in which they are dispersed.
Mica by itself is not a satisfactory nacreous pigment inasmuch as its average index of refraction is about 1.58 which is too close to the index of conventional transparent resinous media of about 1.5-1.59. Excellent nacreous pigments may, however, be provided by the deposition of titanium dioxide or iron oxide coatings on mica flakes.
Linton U.S. Pat. Nos. 3,087,828 and 3,087,829 describe the preparation of titanium dioxide and other metal oxide coated mica nacreous pigments, which optionally can be topped with a layer of another material such as, inter alia, iron and chromium.
Armanini, et al. U.S. Pat. No. 4,146,403 describes iron oxide-coated mica nacreous pigments which are improved by interposing a thin layer of titanium dioxide or aluminum oxide between the iron oxide and the mica. Dark colors and a very good adhesion of the iron oxide layer is obtained.
Pearlescent or nacreous pigments are frequently evaluated by examining or measuring reflectance by means of conventional drawdowns on a hiding power chart. For instance, drawdowns are prepared from a suspension containing 3% pigment in a nitrocellulose lacquer as described, inter alia, in the aforementioned Armanini, et al. patent.
The pearlescent pigments are conventionally used to color various materials. They can be incorporated, for instance, in plastics or coated on a substrate using conventional techniques. In such applications, the pearlescent plastics exhibit a constant pearlescent effect. It has now been surprisingly discovered that certain pearlescent pigments exhibit a unique effect when coated on a color card, a metallic substrate, incorporated into a plastic chip, etc. More particularly, these pigments exhibit are intensely colored, almost phosphorescently colored, and are highly reflective.
This invention is related to new articles exhibiting an intense color and high reflectivity and their preparation. More particularly, the invention relates to a platelet pearlescent pigment having a chromium coating on titanium dioxide platelets and an iron oxide coating on the chromium coating.
The pearlescent pigments used in the present invention are iron oxide-coated chromium-coated titanium dioxide platelet nacreous pigments. These titanium dioxide platelet nacreous pigments are derived from titanium dioxide-coated mica substrate from which the substrate has been removed and are, in the general sense, known. Any known procedures can be used to prepare such pigments. The formulation of coating compositions containing such pigments and the coating of substrates is likewise known.
Titanium dioxide platelets suitable for use in this invention are described, for instance in U.S. Pat. Nos. 4,192,691 and 5,611,851, which are incorporated herein for their teaching of titanium platelets and methods for their manufacture. Such platelets are commonly referred to as xe2x80x9cplaty TiO2xe2x80x9d or xe2x80x9cself supporting TiO2xe2x80x9d and are substantially substrate free, generally containing less than about 20% of substrate based on the total weight of the product. U.S. Pat. No. 4,192,691 employs an aqueous solution of hydrofluoric acid and a mineral acid such as sulfuric acid to dissolve the mica from the pigment. It also discloses and illustrates the use of this dissolving agent to remove the mica from a titanium dioxide-coated mica having a surface layer of either iron or chromium oxide. U.S. Pat. No. 5,611,851 employs a combination of a mineral acid and phosphoric acid followed by an extractive dissolution using an alkali. Although the procedure of U.S. Pat. No. 5,611,851 is preferred, other procedures can be employed to obtain the titanium dioxide platelets used in the present invention.
The TiO2 platelet types useful in this invention can be prepared via the web method, using fluid bed techniques, hydrolyzing organic titanates or homogeneous precipitation employing NaOH/TiCl4. In addition, platelet TiO2 suitable for this invention can be prepared by removing gypsum from TiO2 coated gypsum or by burning off graphite from TiO2 coated graphite. Dissolving glass from a TiO2 coated glass base also provides a substrate useful in this invention. Although there are several avenues for preparing the Ti which then can be coated further, the TiO2 substrate of U.S. Pat. No. 5,611,851 still preferred in order to obtain maximum reflectivity and color purity.
The platelets of titanium dioxide are generally about 1-75 xcexcm in length, preferably about 2-35 xcexcm, and have an interference thickness of about 5-600 nm, preferably about 20-400 nm. By substantially substrate-free is meant that the platelet material can incorporate up to about 20% of the mica substrate and usually incorporate at least 1% mica. The TiO2 is preferably in the rutile crystalline form but can also be in the anatase form.
Providing a titanium dioxide-coated mica with a surface layer of either iron or chromium oxide is known. In broad terms, the material to be coated is brought into contact with a salt of the metal, usually an aqueous solution thereof, under appropriate conditions, e.g. pH, so as to deposit a layer of the metal, followed by calcination. The present invention utilizes such known procedures but differs therefrom in that titanium dioxide platelet is used as the material to be coated, both chromium and iron are deposited thereon and the chromium is deposited first. The chromium layer is preferably partly calcined before the iron is deposited but can be fully calcined if desired. To achieve partial calcination, a lower calcination temperature of about 350-500xc2x0 C., preferably about 425-475xc2x0 C., is employed for about 0.1-5 hours, preferably 0.25-0.75 hour. In general, the chromium will constitute about 2-8%, preferably about 4-5%, of the final calcined product and the iron will constitute about 4-10%, preferably about 6-7%, of the final calcined product.
After the iron is deposited and final calcining, the reflectivity is increased and the interaction between Ti, Cr and Fe returns an intense, almost phosphorescent color. Colors between gold and green, within the interference spectrum, can be prepared. In each case, reflectivity increases with intense color formation. This is the first time such an effect has been recorded or reported, using a substrate-free interference type TiO2 substrate. Compared to prior art, this process has several advantages. It employs standard, pearlescent coating technology without resorting to solvent based reactions or chemical vapor deposition/reduction techniques, it takes full advantage of the high refractive index of TiO2 (2.6-2.9) without mica presence, it does not require costly or impractical equipment, and it produces a full range of colors which can be used in cosmetic/automotive/industrial markets.